Effects of temperature and molecular weight on dissolution of cellulose in NaOH/urea aqueous solution

Dissolution of cellulose having different viscosity-average molecular weight (M _η ) in 7 wt%NaOH/12 wt%urea aqueous solution at temperature from 60 to −12.6°C was investigated with optical microscope, viscosity measurements and wide X-ray diffraction (WXRD). The solubility (S_a) of cellulose in NaOH/urea aqueous solution strongly depended on the temperature, and molecular weight. Their S_a values increased with a decrease in temperature, and cellulose having M _η below 10.0 × 10⁴ could be dissolved completely in NaOH/urea aqueous solution pre-cooled to −12.6°C. The activation energy of dissolution (E_a,s) of the cellulose dissolution was a negative value, suggesting that the cellulose solution state had lower enthalpy than the solid cellulose. The cellulose concentration in this system increased with a decrease of M _η to achieve about 8 wt% for M _η of 3.1 × 10⁴. Moreover, cellulose having 12.7 × 10⁴ could be dissolved completely in the solvent pre-cooled to −12.6°C as its crystallinity (χ _c) decreased from 0.62 to 0.53. We could improve the solubility of cellulose in NaOH/urea aqueous system by changing M _η , χ _c and temperature. In addition, the zero-shear viscosity (η ₀ ) at 0°C for the 4 wt% cellulose solution increased rapidly with an increase of M _η , as a result of the enhancement of the aggregation and entanglement for the relatively long chains.